Ivars Wrote:Imaginary unit is a universal notation for infinitesimals if we do not care about differences they have in different scales.

No no no! The imaginary unit is whereas i is just a letter. Do not confuse the two, one is a letter, one is a number. Feel free to use the letter i for infinitesimals (or for whatever you want), but when you do so, do not call it "imaginary unit" call it "the letter i".